Web theme change after certain period of time by Java Script

We can change Change web theme by using Java script. Just the following java script code have to be include your file.

<script language="javascript">
function getCSS(){

datetoday = new Date();
timenow = datetoday.getTime();
datetoday.setTime(timenow);
thehour = datetoday.getHours();

if(thehour = 20)
display = "tree_twilight.css";
else if(thehour = 17 )
display = "tree_sunset.css";
else if(thehour = 14 )
display = "tree_afternoon.css";
else if(thehour = 11 )
display = "tree_noon.css";
else if(thehour = 7)
display = "tree_morning.css";
else if(thehour = 4 )
display = "tree_sunrise.css";
else if(thehour = 1 )
display = "tree_twilight.css";
else
display = "tree_sunset.css";

var css = '<'; css+='link rel="stylesheet" href='+display + '/'; css+='>';
document.write(css);
}
</script>

<script language="javascript">  
getCSS();
</script>